1. Field of the Invention
The present invention relates to a soft switching DC motor driver and a related driving circuit for avoiding reverse current, and more particularly, to a soft switching DC motor driver and a related driving circuit for avoiding reverse current by using operational amplifiers for fixing output voltages.
2. Description of the Prior Art
A DC motor driver is a necessary power transformation device in modern industries and information society. The DC motor is capable of transforming electricity into kinetic energy required for driving devices. Conventional motors include DC motors, AC motors, and stepping motors. DC motors and AC motors are often applied in products not requiring delicate manipulations. For example, blades of an electric fan are rotated with a DC motor or an AC motor. As the technology of digital products grows, a rotation rate of a DC motor or an AC motor is required to be faster and faster. However, with a high rotation rate of a motor, the current of the motor cannot be consumed completely. The unconsumed and therefore remaining currents reversely flow to a corresponding power supply. This scenario leads to damages of controllers and drivers of the motor. Therefore, the damages caused by reverse current under a high rotation rate of the motor have to be avoided.
Please refer to FIG. 1, which is a diagram of a prior art soft switching DC motor driver 10. The soft switching DC motor driver 10 comprises a power supply 12, an input capacitor C1, a hall sensor 16, a first amplifier AMP1, a second amplifier AMP2, and a full-bridge driver circuit 14. The power supply 12 is utilized for generating an input voltage Vin. The input capacitor C1 is coupled to the power supply 12. A voltage difference between both terminals of the input capacitor C1 is a supply voltage VDD. The Hall sensor 16 has a first output end 162 for generating a first timing control signal H+, and a second output end 164 for generating a second timing control signal H−. The first amplifier AMP1 has a first input end 102 coupled to the first output end 162 of the Hall sensor 16, a second input end 104 coupled to the second output end 164 of the Hall sensor 16, a first output end 106, and a second output end 108. The first amplifier AMP1 is utilized for amplifying signals inputted from the first input end 102 and the second input end 104. The second amplifier AMP2 has a first input end 112 coupled to the second output end 164 of the Hall sensor 16, a second input end 114 coupled to the first output end 162 of the Hall sensor 16, a first output end 116, and a second output end 118. The second amplifier AMP2 is utilized for amplifying signals inputted from the first input end 112 and the second input end 114. The full-bridge driver circuit 14 has an input end 142 coupled to the power supply 12 and the input capacitor C1, and the voltage at the input end 142 is the supply voltage VDD. The full-bridge driver circuit 14 comprises a first switch SW1, a second switch SW2, a third switch SW3, a fourth switch SW4. The inductor L is modeled as a motor. The first switch SW1 has a control terminal 132 coupled to the first output end 106 of the first amplifier AMP1, an input end 134 coupled to the power supply 12 and the input capacitor C1, and an output end 136 for generating a first output voltage Vout1. The second switch SW2 has a control terminal 152 coupled to the second output end 108 of the first amplifier AMP1, an input end 154 coupled to ground, and an output end 156 coupled to the output end 136 of the first switch SW1. The third switch SW3 has a control terminal 172 coupled to the first output end 116 of the second amplifier AMP2, an output end 174 coupled to the power supply 12 and the input capacitor C1, and an output end 176 for generating a second output voltage Vout2. The fourth switch SW4 has a control terminal 192 coupled to the second output end 118 of the second amplifier AMP2, an input end 194 coupled to ground, and an output end 196 coupled to the output end 176 of the third switch SW3. The inductor L has a first terminal 182 coupled to the first switch SW1 and the second switch SW2, and a second terminal 184 coupled to the third switch SW3 and the fourth switch SW4. The soft switching DC motor driver 10 further comprises a protecting diode D1 coupled to the power supply 12 and the input capacitor C1 for protecting the power supply 12 and for preventing reverse current, which may burn down the entire integrated circuit. The first switch SW1, the second switch SW2, the third switch SW3, and the fourth switch SW4 may be metal-oxide semiconductor transistors, the first switch SW1 and the third switch SW3 are P-type metal-oxide semiconductor transistors, and the second switch SW2 and the fourth switch SW4 are N-type metal-oxide semiconductor transistors. The first switch SW1, the second switch SW2, the third switch SW3, and the fourth switch SW4 may also be bipolar-junction transistors, the first switch SW1 and the third switch SW3 are npn bipolar-junction transistors, and the second switch SW2 and the fourth switch SW4 are pnp bipolar-junction transistors.
Please refer to FIG. 1 and FIG. 2. FIG. 2 is a waveform diagram of the signals shown in FIG. 1. The soft switching DC motor driver 10 controls the switches of the full-bridge driver circuit 14 by soft switching driving techniques. Therefore, waveforms of the first output voltage Vout1 and the second output voltage Vout2 are trapezoidal waves for mitigating high-frequency voltage pulse of the soft switching DC motor driver 10 during transitions and a voltage impulse caused by reverse current. Therefore, low noises of the soft switching DC motor driver 10 is achieved and the reliability of the soft switching DC motor driver 10 is also enhanced.
Please refer to FIG. 2 again. The first timing control signal H+ and the second timing control signal H−, which are outputted by the Hall sensor 16, are utilized for controlling the first switch SW1, the second switch SW2, the third switch SW3, and the fourth switch SW4. When the first timing control signal H+ is low and the second timing control signal H− is high, the first switch SW1 and the fourth switch SW4 are turned on, and the second switch SW2 and the third switch SW3 are turned off. An inductance current flows IL from the first output voltage Vout1 to the second output voltage Vout2, at this time, the first output voltage Vout1 is high, and the second output voltage Vout2 is low. During the transition of the first timing control signal H+ and the second timing control signal H−, the second switch SW2 and the fourth switch SW4 are turned on, and the first switch SW1 and the third switch SW3 are turned off. The inductance current IL weakens gradually through the second switch SW2 and the fourth switch SW4. At this time, the waveform of the first output voltage Vout1 descends linearly during a transient whereas the waveform of the second output voltage Vout2 ascends linearly during the transient. When the first timing control signal H+ is high and the second timing control signal H− is low, the second switch SW2 and the third switch SW3 are turned on, and the first switch SW1 and the fourth switch SW4 are turned off. The inductance current IL flows from the second output voltage Vout2 to the first output voltage Vout1, at this time, the first output voltage Vout1 is low, and the second output voltage Vout2 is high.
Please refer to FIG. 3, which is a waveform diagram of the signals of FIG. 1 when a high rotation rate of the soft switching DC motor results in a voltage impulse. During the first stage, the first timing control signal H+ is low, and the second timing control signal H− is high. The inductance current IL flows from the first output voltage Vout1 to the second output voltage Vout2, at this time, the first output voltage Vout1 is high, and the second output voltage Vout2 is low. During the second stage and the transition of the first timing control signal H+ and the second timing control signal H−, the inductance current IL weakens gradually through the second switch SW2 and the fourth switch SW4. However, since the rotation rate of the soft switching DC motor is high, the inductance current IL cannot be weakened to be zero after the transition of the switches. Therefore, during the third stage, the inductance current IL flows reversely to the supply voltage VDD through the second switch SW2 and the fourth switch SW4, and charges the input capacitor to result in a voltage impulse. As shown in FIG. 3, the magnitude of the voltage impulse depends on the magnitude of the reverse current flowing into the input capacitor C1 and the capacitance of the input capacitor C1. During the fourth stage when the first timing control signal H+ turns to high and the second timing control signal H− turns to low, the inductance current IL flows from the second output voltage Vout2 to the fist output voltage Vout1, at this time, the first output voltage Vout1 is low, and the second output voltage Vout2 is high.
Please refer to FIG. 4, which is a diagram illustrating the flow of the inductance current IL during the first stage shown in FIG. 3. During the first stage, the first amplifier AMP1 and the second amplifier AMP2 are saturated, and the first switch SW1 and the fourth switch SW4 are fully on. The inductance current IL flows from the first output voltage Vout1 to the second output voltage Vout2.
Please refer to FIG. 5, which is a diagram illustrating the flow of the inductance current IL during the second stage shown in FIG. 3. During the second stage, a feedback loop of both the first amplifier AMP1 and the second amplifier AMP2 begins working, then the first output voltage Vout1 descends linearly whereas the second output voltage Vout2 ascends linearly. At this time, the inductance current IL weakens gradually, and the first switch SW1 and the fourth switch SW4 are turned on. The inductance current IL continues to flow from the first output voltage Vout1 to the second output voltage Vout2.
Please refer to FIG. 6, which is a diagram illustrating the flow of the inductance current IL during the third stage shown in FIG. 3. During the third stage and when the first output voltage Vout1 falls below −0.7 volts, since diode of the second switch SW2 is turned on, the feed back loop due to the first amplifier AMP1 is broken, and the switch SW1 no longer provides current to the soft switching DC motor driver 10, therefore, the first switch SW1 is turned off whereas the second switch SW2 is fully on. Similarly, when the second output voltage Vout2 increases over (VDD+0.7) volts, the fourth switch SW4 is turned off whereas the second switch SW2 is turned on. At this time, the inductance current IL charges the input capacitor C1 to increase the supply voltage VDD and to result in a voltage impulse.
Please refer to FIG. 7, which is a diagram illustrating the flow of the inductance current IL during the fourth stage shown in FIG. 3. During the fourth stage, since the inductance current IL has weakened to be zero, the second switch SW2 and the third switch SW3 are turned on whereas the first switch SW1 and the fourth switch SW4 are turned off. The inductance current IL flows from the second output voltage Vout2 to the first output voltage Vout1.
For a DC motor having a low rotation rate, controlling the switches of the full-bridge driver circuit 14 with the soft switching driving techniques may mitigate the high frequency voltage pulse of the soft switching DC motor driver 10 during the transition and the voltage impulse caused by the reverse current. However, in modern applications, the rotation rate of a modern motor is ever increasing. When the rotation rate of the motor exceeds a limit, the inductance current IL has not weakened to be zero after the transition of the switches. At this time, the inductance current IL flows reversely to the supply voltage VDD through the second switch SW2 and the fourth switch SW4, and charges the input capacitor C1 to result in the voltage impulse. Therefore, the controller and the driver of the soft switching DC motor driver 10 would be damaged or burned down, the power supply 12 may be burned down also, and the reliability and the effective operational range of the system of the soft switching DC motor driver 10 would be degraded.